1. Field of the Invention
This invention relates generally to vacuum pumps and more particularly to ion pump elements.
2. Related Art
An ion pump (also referred to as a sputter ion pump) is a type of known vacuum capture pump capable of reaching pressures as low as 10−11 mbar under ideal conditions. An ion pump is a device that ionizes gas within a vessel (to which the ion pump is attached) and employs a strong electrical potential, typically 3 kV to 7 kV, that allows the gas ions to accelerate into and be captured by a solid electrode and its residue.
The basic element of a known ion pump is a Penning trap. Penning traps are devices for the storage of charged particles using a homogeneous static magnetic field and a spatially inhomogeneous static electric field. Penning traps use a strong homogeneous axial magnetic field to confine particles radially and a quadrupole electric field to confine the particles axially. In FIG. 1, a perspective view of an example of an implementation of a known Penning trap 100 is shown.
The static electric potential can be generated using a set of three electrodes: a ring 102 and two end-caps 104 and 106 between a magnet 108. In this example, the ring 102 is an anode element, such as a cylindrical anode of stainless steel, and the end-caps 104 and 106 are cathodes. For trapping of ions, the end-cap electrodes 104 and 106 are kept at a negative potential relative to the cylindrical anode 102. This potential produces a saddle point in the center of the Penning trap 100, which traps ions along the trap axial direction 110. The electric field causes ions to oscillate along the trap axis 110. The magnetic field in combination with the electric field causes charged particles to move in the radial plane 112 with a motion which traces out a helix.
In FIG. 2, a side-view of the Penning trap 100 of FIG. 1 is shown in combination with the vessel 200 that has an inlet 202. The cathode plates 104 and 106 are shown positioned on both sides of one of the anode cylinders 102. It is appreciated that while only one anode cylinder 102 is shown for convenience, the description extends to a plurality of anode cylinders 102. Typically, the anode 102 is made of stainless steel, aluminum or other similar metals, which serves as the gettering material. A magnetic field 204 is oriented along the axis 206 of the anode 102. Electrons 208 are emitted from the cathode 104 and 106 due to the action of an electric field 210 and, due to the presence of the magnetic field 204, the electrons 208 move in long helical trajectories 212 which improves the chances of collision with gas molecules 214 inside the Penning cell 100 that are introduced via the inlet 202.
The usual result of a collision of a gas molecules 214 with the electron 208 is the creation of a positive ion 216 that is accelerated to some voltage potential by the anode voltage and moves almost directly in the direction 218 to the cathode 106. The influence of the magnetic field 204 is small because of the ion's relatively large atomic mass compared to the electron mass.
In this example, the cathodes 104 and 106 may be of titanium (tantalum, other related alloys, or other getterable metals). In the case of cathodes 104 and 106 being made of titanium, ions 216 impacting on the titanium cathode surface sputter titanium atoms (or molecules) 220 in a direction 222 away from the cathode 106 forming a getter film on the neighboring surfaces and stable chemical compounds with the reactive or “getterable” gas particles (e.g. CO, CO2, H2, N2, O2). This pumping effect is very selective for the different types of gas molecules 214 and is the dominating effect with ion pumps. The number of sputtered titanium molecules 220 is proportional to the pressure inside the ion pump. The sputtering rate depends on the ratio of the mass of the bombarding molecules 216 and the mass of the cathode material 220.
In an example of operation, a swirling cloud of electrons 208 produced by a Penning discharge within the Penning trap 100 are temporarily stored in the anode region 224 of the Penning trap 100. These electrons 208 ionize incoming gas atoms and molecules 214. The resultant swirling ions 216 are accelerated to strike the chemically active cathodes 104 and 106. On impact the accelerated ions 216 will either become buried within the cathode 104 and 106 or sputter cathode material 220 onto the walls 224 of the ion pump. The freshly sputtered chemically active cathode material 220 acts as a getter that then evacuates the gas by both chemisorption and physisorption resulting in a net pumping action.
Both the pumping rate and capacity of such capture methods are dependent on the specific gas molecules 214 being collected and the cathode material absorbing it. Some gas molecules 214, such as carbon monoxide, will chemically bind to the surface of a cathode material. Others, such as hydrogen, will diffuse into the metallic structure.
A problem with known Penning traps is that the anodes 102 are typically assembled utilizing spot welding techniques. Spot welding is a process in which the contacting metal surfaces of the anode 102 are joined by the heat obtained from resistance to electric current. These contacting metal surfaces are held together under pressure exerted by electrodes where the electrodes are typically two shaped copper alloy electrodes to concentrate welding current into a small “spot” (or spots) and to simultaneously clamp the sheets together. By forcing a large current through the spot(s) it melts the metal and form the weld.
Unfortunately, this welding process causes the introduction of impurities (through particles, contamination and/or oxidation of the anode material) into the metal of the welded anode 102. These impurities cause the ion pump to operate at less efficiency than if no impurities are introduced by introducing particles that can create leakage currents when the ion pump in operating. The problem is increased if vacuum fired cathodes are desired because generally these situations typically reach extremely low pressure ranges where the ion current is comparable to the leakage current. As such, there is a need for a process for producing anode elements that do not have the impurities produced by spot welding techniques.